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The shapes of galaxies can be quantified by ratios of their quadrupolemoments. For faint galaxies, observational noise can make the denominator closeto zero, so the ratios become ill-defined. Knowledge of these ratios (i.e.their measured standard deviation) is commonly used to assess the efficiency ofweak gravitational lensing surveys. Since the requirements cannot be formallytested for faint galaxies, we explore two complementary mitigation strategies.In many weak lensing contexts, the most problematic sources can be removed by acut in measured size. We investigate how a size cuts affects the requiredprecision of the charge transfer inefficiency model and find slightly widertolerance margins compared to the full size distribution. However, subtlebiases in the data analysis chain may be introduced. Instead, as our secondstrategy, we propose requirements directly on the quadrupole momentsthemselves. To optimally exploit a Stage-IV dark energy survey, we find thatthe mean and standard deviation of a population of galaxies' quadrupole momentsmust to be known to better than $1.4\times10^{-3}$ arcsec$^{2}$, or the Stokesparameters to $1.9\times10^{-3}$ arcsec$^2$. This testable requirement can nowform the basis for future performance validation, or for proportioning therequirements between subsystems to ensure unbiased cosmological parameterinference.

Problems using ratios of galaxy shape moments on requirements for weak lensing surveys